Methods and devices for hardening workpieces by means of carburizing are known in the prior art.
DE 103 22 255 A1 discloses a method for carburizing steel pans at temperatures above 930° C. with a carbon donor gas within a treatment chamber which can be evacuated, wherein nitrogen-releasing gas, such as ammonia, is fed into the treatment chamber both during the heating phase and during the diffusion phase.
DE 103 59 554 134 describes a method for carburizing metallic workpieces in a vacuum furnace, wherein the furnace atmosphere contains a carbon carrier which, under the process conditions for the carburizing, is cleaved with the release of pure carbon, wherein the carbon carrier is supplied in pulsed fashion, each carburizing pulse is followed by a diffusion pause and the amount of hydrocarbon to be supplied during a carburizing pulse is varied in such a way that it is adapted to the present capacity of the material, to which end the volumetric flow rate of acetylene at the start of each carburizing pulse is dimensioned to be high, and the concentration of hydrogen and/or acetylene and/or total carbon which prevails in the furnace atmosphere or in the off-gas is measured, and according to this the volumetric flow rate of acetylene is appropriately lowered.
DE 10 2006 048 434 A1 relates to a carburizing method which is performed in a protective gas or treatment atmosphere in a heat-treatment furnace, wherein an alcohol and carbon dioxide are introduced into the heat-treatment furnace and reacted chemically. Ethanol and carbon dioxide are introduced into the heat-treatment furnace, wherein the ratio of introduced ethanol to introduced carbon dioxide is preferably 1:0.96. A heat-treatment atmosphere which is produced in such a manner is suitable in particular for the carburization and carburization-neutral annealing of metallic materials, for example iron materials.
DE 10 2007 038 991 A1 describes a rotary hearth furnace for the heat treatment of workpieces, in particular for the gas carburization of metallic workpieces, comprising a furnace chamber, a rotary hearth which bounds the furnace chamber at the bottom, an outer wall which laterally surrounds the furnace chamber and a cover plate which bounds the furnace chamber at the top, wherein the furnace chamber is subdivided into at least two treatment zones with inner walls, which extend radially with respect to an axis of rotation of the rotary plate. For the treatment of workpieces, a plurality of radially chargeable racks which are oriented radially with respect to the axis of rotation of the rotary plate and are intended to receive workpieces or workpiece carriers are arranged on the rotary plate, wherein each inner wall has a passage which is shaped in a manner complementary to the racks and through which the racks can be guided through the respective inner wall when the rotary plate rotates in the circumferential direction.
DE 10 2007 047 074 A1 discloses a method for carburizing steel workpieces, in particular workpieces having outer and inner surfaces, wherein the workpiece is held at a temperature in the range of 850 to 1050° C. in an atmosphere containing gaseous hydrocarbon. At least two different gaseous hydrocarbons are used and/or the workpiece is held alternately during a carburizing pulse in the atmosphere containing the gaseous hydrocarbon and during a diffusion phase in an atmosphere which is free of hydrocarbon.
The methods which are known in the prior art have one or more of the following disadvantages:                the temperature which is required to harden workpieces by means of carburization is more than 850° C. with times of more than 45 min usually being required for heating. In order to achieve a sufficient productivity or a high throughput of workpieces, the carburization is effected in batch-wise fashion with a large number of workpieces, which are arranged in a plurality of layers arranged one above another in a charging rack. By way of example, a charging rack having 10 grills is loaded in total with 160 hollow wheels made of a 28Cr4 alloy (in accordance with ASTM 5130), with 16 hollow wheels being arranged alongside one another on each of the 10 grills. Typical charges or charging racks have a dimension in the range of 400 mm up to 2000 mm in each of the three spatial directions. Here and in the text which follows, this conventional type of charging is also denoted by the term “3D charge”. In the production sequence, the carburization follows the substantially serial machining (the so-called soft machining). To this end, provision is made of buffer regions, in which the soft-machined workpieces are collected until a 3D charge for carburization is completed. The carburization of 3D charges takes up considerable areas both for the heating furnace and for the buffer region. In addition, it interrupts the quasi-continuous flow of the machining and leads to additional expenditure for logistics. Thus, the buffering of 3D charges requires the manual handling of workpieces, because robot systems which are suitable for this purpose cannot be used for technical and economical reasons;        the carburization of 3D charges leads to the increased formation of carbon-containing residues, which can contaminate the workpieces and also the surrounding production line;        workpieces carburized in 3D charges generally experience considerable thermal distortion, which makes complex remachining (the so-called hard machining necessary;        workpieces carburized in 3D charges have a broad variation in characteristic properties, such as the carburization depth, the surface carbon content and the core hardness, and therefore it is not possible to improve characteristic quality values which are influenced directly or indirectly thereby, for example the slip or frictional loss of a mechanical gearing mechanism which is composed of carburized parts.        